It's because a 3 is worth twice as much as a 2 when you play with 1's and 2's, and since there's no quality team defense going on in pick up games, 3s are too easy to get open for.

Alright, again, I'm trying not to overthink this (I'm bound to anyway), but is the idea that 2 being twice as 1 where 3 is not twice as 2 the most relevant aspect of this difference? I feel like the true relevant portion would be that "three pointers" in each instance still only award one bonus point beyond a normal score.

Granted, the 3-to-2 versus 2-to-1 does still make a difference, as a game of 21 by 2's and 3's could be won by making 7 "three pointers", where a game to 11 by 2's and 1's could be won by making only 6 "three pointers". In both instances, a non-three-pointer would have to be made 11 times in order to reach the final point totals of either 11 or 21. I'm just not sure having to make six threes to win instead of seven would make enough of a difference to justify people beginning to shoot threes wildly.